Sorry, I forgot about the subform. You can set the values like this:
[VBA]
Forms!FormNewOrder.NameOfSubformControl!ProductId = Me.ProductId
[/VBA]

Note that NameOfSubformControl is the name that appears when you open frmNewOrder in design view, right-click on one of the outside borders of the subform, select Properties (Properties dialog should be titled Subform/Subreport), and open the Other tab. The default naming scheme for subforms will look something like "Child1."